Different principles are known for driving stepping motors of a variety of winding types. These principles are summarized in an article in the publication "Computer Design" Mar. 19, 1971 entitled "Drive System for Small or Large Angle PM Stepping Motors " by Thomas E. Beling. In this paper, six possibilities for control are mentioned. The first is as follows:
1. The motor operating voltage for the stepping drive is selected that with the winding resistance nominal current occurs. This, however, results, in a relatively great time constant. The remaining possibilites are designs which obtain as steep a current rise as possible. To accomplish this they use operating voltages substantially higher than the nominal voltage which would be called for to obtain a nominal current at a give winding resistance. In such cases the winding current must be limited. Thus, in each of the remaining cases an excess or overvoltage is employed and the control schemes deal with means for limiting the current. These are as follows: PA1 2. A series resistance is used for the limiting the current. The major disadvantage of this is a high power loss. PA1 3. The current is limited by a series control. This also leads to a high power loss. PA1 4. An on-off or chopper regulator is provided. PA1 5. A bi-level drive is used in which the voltage is switched between two operating voltages V.sub.1 and V.sub.2 of different magnitude. The higher operating magnitude is used to overexcite the motor initially and after a constant time is switched to a lower V.sub.2 which is equal to the nominal current times the winding resistance, i.e. I.sub.N .times. R.sub.L through the use of a time delay stage, and PA1 6. A bi-level system is used in which, upon reaching the nominal current the higher voltage V.sub.1 is switched to the lower voltage V.sub.2 through the use of a trigger and the nominal current then flows in the windings.
Of the above noted possibilities (4 ) - (6 ) have a lower power loss than the earlier possibilities and are therefore preferable.
However, to reduce the average power loss still further it is required that during standstill or stopping of the motor i.e. during control pulse interval, the motor winding current be reduced to the point where it is equal only to that necessary to generate a holding torque for the motor. This is particularly important where a great number of such motors and drives with stepping motors are used in a system. The required holding current for a stepping motor is considerably smaller than the nominal current since it is used only to suppress an accidental angular displacement of the motor during the control pulse intervals. It is necessary, however; if a holding current was not present the motor could be moved slightly.